1. Field of the Invention
The present invention relates to probing and analysis of semiconductor integrated circuits and in particular, to a probe for extracting dopant profiles from semiconductor structures.
2. Description of the Related Art
The most commonly used method for the generation of resistivity or carrier concentration profiles perpendicular to the surface of a processed semiconductor wafer is the Spreading Resistance Probe (SRP) method. See R. G. Mazur and D. H. Dickey, "A Spreading Resistance Technique for Resistivity Measurements in Si", J. Electrochem. Soc., 113, 255 (1966).
The conventional SRP method is described in various reference works. See for example: D. K. Schroder, Semiconductor Material and Device Characterization, Wiley, New York (1990); ASTM Standard F672 "Standard Method for Measuring Resistivity Profile Perpendicular the Surface of a Silicon Wafer Using a Spreading Resistance Probe", 1988 Annual Book of ASTM Standards, American Soc. of Test. Mater. Conf., Philadelphia (1988); R. Brennan and D. Dickey, "Determination of Diffusion Characteristics Using Two-and Four-Point Probe Measurements, Solid-State Technology, 27, 125 (1984); and R. J. Hillard, R. G. Mazur, H. L. Berkovits, and P. Rai-Choudhury, "Profiling of Silicon and III-IV Compounds by Point-Contact Techniques", Solid-State Technology, 32, 119 (1989).
As shown generally in FIG. 1, and in greater detail in FIG. 2, the conventional SRP method uses a two-point resistance measurement on a bevelled surface. The total resistance R.sub.T measured between the probes has several components, i e., EQU R.sub.T =2R.sub.c +2R.sub.sp +R.sub.s .about.2R.sub.sp, (1)
where R.sub.c is the contact resistance, R.sub.sp is the spreading resistance and R.sub.s is the semiconductor resistance between the probe contacts. The spreading resistance R.sub.sp accounts for the resistance encountered by the current I when it flows from the metal probe into the semiconductor material.
For analytical purposes, the probe can be approximated as a highly conductive, cylindrical bar that is placed into non-indenting contact with a semi-infinite semiconductor block. It has been demonstrated (see Hillard et al., Ibid) that such an arrangement yields a spreading resistance EQU R.sub.sp =.rho./(4r), (2)
where .rho. is the semiconductor resistivity and r is the probe radius.
It can easily be seen that if the probe radius r is made small enough and the contact resistance R.sub.c is minimized, then the spreading resistance R.sub.sp becomes the dominant component of the total resistance R.sub.T, as reflected in Eq. (1). Moreover, about 80% of the potential drop due to the spreading phenomenon occurs within a distance of five times the contact radius, which makes the SRP method a good tool for local resistivity characterization.
The application of this method to actual resistivity measurements involves complex calculations, the result of which is a multiplicative correction factor to Eq (2). Furthermore, application of the method to non-uniform doping profiles involves mathematical and physical analysis making up a specialized literature that has been generated by a confined circle of authors. See, for example, J. R. Ehrstein, "Emerging Semiconductor Technology, ASTM STP 960 (Edited by D. C. Gupta and P. H. Langer), American Soc. Test. Mater. Conf. Philadelphia (1986); and S. C. Choo, M. S. Leong, C. B. Liem and K. C. Kong, "Extraction of Semiconductor Dopant Profiles from Spreading Resistance Data: An Inverse Problem", Solid-State Electronics, 33, 783, (1990).
Most recently, space-charge effects have been observed to influence SRP measurements (specifically in deep profiles). These effects are accounted for utilizing advanced simulation tools. See W. B. Grabowski, "Simulation of SRP Bevel Effects", Application Note 1001, Technology Modeling Associates, Inc., Palo Alto (1986).
Although the SRP method is the most commonly used procedure for measuring processed semiconductor wafers, it is frequently desireable to measure actual semiconductor devices rather than a processed semiconductor wafer. The Mazur/Dickey SRP method requires the wafer to have specially devised rectangular patterns of 1,000 .mu.m.times.100 .mu.m (preferred dimensions), or at least 200 .mu.m.times.40 .mu.m. See "How Big a Pattern Do We Need for Spread Resistance Analysis", Solecon Labs Technical Note, Jun. 7, 1990.
Since most semiconductor devices are substantially smaller than the minimum sizes required by the Mazur/Dickey SRP method, the Mazur/Dickey SRP method cannot be used to measure an actual semiconductor device except where the largest semiconductor devices are involved.
Additionally, the physical size of the SRP probes, which are typically several micrometers in diameter, are physically so large that it is impossible to profile very shallow junctions due to the mechanical punch through of the probes. Further, the conventional SRP technique only measures the junction depth and not the junction width.
Other techniques for extracting dopant profile are also available. The closest to satisfying the small-area requirements is the Secondary-Ion Mass Spectroscopy (SIMS) technique. See S. M. Sze, VLSI Technology, McGraw-Hill, New York, 1988. Although, theoretically, the ion-beam spot could be confined to a few micrometers in diameter, the SIMS method appears to be impractical and cost-inefficient under normal conditions. Moreover, the presence of the same doping element, i.e., boron, in the field regions (under the field oxide) poses serious discrimination problems if the ion-beam spot intrudes into the respective regions.
It appears that no practical method is currently available for the extraction of impurity profiles from individual devices on a semiconductor wafer. Thus, there is a need for a device which can take profile measurements on individual devices.